The decomposition of litter of two tree species Graffenrieda emarginata (Melastomataceae), Purdiaea nutans (Cyrillaceae) and the mixture of both was investigated in a tropical montane rain forest in southern Ecuador at two different altitudes (1850 and 2280 m). The two litter types differed strongly in nitrogen concentration, suggesting that G. emarginata (1.21% N) decomposes faster than P. nutans (0.73% N). To study the effect of soil micro-arthropods on the decomposition process, litterbags with mesh-size of 48 μm, excluding soil micro-arthropods, and 1 mm, allowing colonization by soil micro-arthropods, were used. Litter mass loss was measured after 2, 6 and 12 mo exposure in the field; further, microbial biomass and micro-arthropod colonization of the litter were investigated after 2 and 12 mo. Generally, litter decomposed faster at 1850 m than at 2280 m (60% and 76% dry mass remaining after 12 mo, respectively); G. emarginata and mixed litter decomposed faster than P. nutans litter. After 12 mo mixed litter decomposed faster (65% of dry mass remaining) than both individual litter species (70% and 71% of dry mass of G. emarginata and P. nutans litter remaining, respectively) indicating that non-additive effects contributed to litter decomposition. Microbial biomass increased during the experiment and was higher at 1850 m than at 2280 m. The most abundant micro-arthropods in both litter types were oribatid mites followed by Collembola, Gamasina, Uropodina and Prostigmata + Astigmata. Micro-arthropods were generally more abundant at 1850 m suggesting higher biotic activity at lower altitudes. Soil micro-arthropods contributed little to decomposition processes indicating that litter decomposition is mainly due to micro-organisms.

Most researchers assume litter decomposition to take place at the site where litterfall occurs, mostly by soil micro-organisms and microfauna of less than 1 mm in size. Accordingly, the litterbag method has become the standard technique for evaluating litter decomposition rates (Schlesinger 1985, Wedderburn & Carter 1999). However, there is evidence indicating that at least in tropical arid ecosystems litter can be removed from the site in substantial amounts by animals, including large vertebrates. For example, litter is removed for forage or as building material for nests by ants (particularly leaf-cutting ants) and termites (Bucher 1982, Scholes & Walker 1993). Moreover, ruminant species (including domestic cattle and goats) supplement their food intake by feeding on falling leaf litter during the dry season in African savannas (Owen-Smith & Cooper 1987) and in the Chaco savannas of Argentina (Morello & Saravia-Toledo 1959). These observations suggest that litter removal may be an important component of the decomposition process, particularly in tropical semi-arid environments where lack of soil moisture decreases microbial activity, increasing litter availability for herbivores.